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Date of Award

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RU Laboratory

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Abstract

Embryology aims at understanding how a single fertilized cell develops into a complex multicellular organism. Initially the embryo is no more than a ball of cells where the three primordial layers, the ectoderm, mesoderm and the endoderm are one on top of the other. The three germ layers will go on to form all the tissues and organs of the embryo. For example, the ectoderm will give rise to epidermis and the nervous system; the mesoderm to muscles, the skeletal system, the dermis or inner layer of the skin, the circulatory, excretory, and reproductive systems; and, finally, the endoderm will give rise to the inner lining of the alimentary canal and the structures derived from it, such as lungs, the liver, pancreas, and the bladder. Correct positioning of the germ layers paves the way for the inductive interactions that are the hallmark of both axis determination and organogenesis. Therefore, the formation of the body plan requires highly integrated and regulated cell movements. The study of these movements is central to the field of embryology.Historically, the amphibian gastrula became one of the predominant models for experimental embryologists. This was partially due to the major influence of studies that lead to the eventual discovery of the organizer by Spemann and Mangold in 1924. After decades of research there is an imposing literature on the subject of inductive interactions in the amphibian and other embryos but the investigation of the movements leading from the relatively simple architecture of a blastula to the advanced and highly complex architecture of the late gastrula has been lacking. Perhaps it is not surprising, given the difficulties of studying these movements, that after almost a century of research fundamental questions still have not been answered. Haeckel first proposed the name "gastrula" in 1872, and although there was a long debate concerning the movements leading to the formation of the gastrula structures, the first experimental evidence for epibolic and inward morphogenetic movements was provided by Kopsch in 1895. Epiboly refers to the intercalation of cells in the animal cap (Figure IA, B and D ) while inward movements are the movements that lead to the internalization of the mesoderm, which is now referred to as involution (Figure 1 compares the location of the orange colored mesoderm at F stage 9 with that of G stage 10). The morphogenetic movements involved in gastrulation were later described by Vogt (1925, 1929) and then studied by Holtfreter (1943, 1944) and, more recently, by Keller(Gerhart and Keller 1986; Keller 1991; Weliky, Minsuk et al. 1991; Wilson and Keller 1991; Keller, Shih et al. 1992). The vast majority of this work was descriptive and only recently have we begun to gain some molecular insight regarding the pathways that are involved in specific morphogenetic movements.

Comments

A thesis presented to the faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy.